Systems for monitoring the movements, if any, of engineering construction work parts are known which consist of an assembly made up on the one hand of a motorized theodolite and on the other hand of targets associated with the theodolite, these targets being on the one hand positioned on parts that are not liable to be affected by movements, that is to say reference targets and, on the other hand, monitoring targets that are fixed at suitable locations on the construction work parts to be monitored. The position of the reference targets may be known with high accuracy. The theodolite or similar apparatus makes it possible, by successively sighting the reference targets and the monitoring targets, to determine the coordinates in space of the reference targets and of the monitoring targets. It is indeed understood that, because the theodolite must usually be placed in the area liable to be affected by movements, it is necessary to be able to accurately determine, on the basis of the reference targets, the position of the theodolite and consequently the position of the monitoring targets.
As is well known, the theodolite is controlled so as to successively and periodically sight the reference targets and the monitoring targets, measuring for each target its polar coordinates: distance, horizontal angle, vertical angle. Usually, the theodolite is equipped with a laser beam generator and the reference and monitoring targets comprise a prism which returns the laser beam to the theodolite when the latter is correctly pointed at the target. It is thus possible to measure the polar coordinates of the reference targets and of the monitoring targets with respect to the theodolite, the Cartesian coordinates of the monitoring targets being calculated by the monitoring centre.
By comparing the coordinates of the monitoring targets for successive instants of measurement of coordinates, it is possible to detect movements, if any, even if the latter are of very small amplitude. Indeed, in order to have a reliable monitoring system, it is necessary to be able to detect displacements or movements of the order of a millimetre.
Although theodolites may have a relatively big range, it may happen, in certain situations, that the possibilities for setting up the theodolite do not make it possible, with the aid of a single theodolite, to perform the monitoring of all the construction work or of all the buildings. This is especially true in an urban area.
Furthermore, it is not always possible, in the environment of the monitoring area and in particular in the case of an urban environment, for there to be areas that are not liable to be affected by movements which allow the placement of reference targets in such a way that these targets allow the determination in three dimensions of the position of the theodolite.
There therefore exists a genuine need for a system for monitoring the movements, if any, of construction work parts which makes it possible to efficiently monitor the various parts of an engineering construction work of considerable dimensions and under conditions in which it is difficult to set up monitoring or reference targets and in particular in an urban setting.